1. Field of the Invention
The present invention relates to a continuous cast slab, which is Ni-added steel produced by using a vertical-bending type or a bow-type continuous casting machine and in which the appearance of surface cracks is restrained, and to a producing method therefore.
This application is a national stage application of International Application No. PCT/JP2009/062808, filed Jul. 15, 2009, which claims priority to Japanese Patent Application No. 2008-183909, filed on Jul. 15, 2008, and the content of which is incorporated herein by reference
2. Description of Related Art
In general, Ni is added to steel in order to improve the toughness of the steel. However, when the Ni-added steel is cast by using a vertical-bending type or a bow-type continuous casting machine, a crack may appear on the surface of the cast slab. In this case, it is necessary to carry out a grinding treatment and the like as a post-process and as a result, the number of processes increases. Accordingly, it is necessary to prevent the appearance of the surface crack on the cast slab in order to improve the productivity of the Ni-added steel.
As a means for solving such a problem, in Japanese Unexamined Patent Application, First Publication No. H09-47854, there is disclosed a method for restraining surface cracks of a cast slab obtained by continuously casting steel. According to the restraining method, the necessary time for drawing a cast slab from a meniscus portion of molten steel in a mold to the lower end of the mold is set to within 1 minute, and the secondary cooling is carried out immediately after the drawing to cool down the surface temperature of the cast slab to the A3 transformation temperature or lower within 1 minute. In addition, the surface of the cast slab at the bending point and the straightening point is reheated up to 850° C. or higher after cooling the surface of the cast slab to the A3 transformation temperature or lower. According to the restraining method, straightening of the cast slab can be finished within 20 minutes after the molten steel passes through a meniscus in the mold.
In Japanese Unexamined Patent Application, First Publication No. 2002-307149, there is disclosed a continuous casting method described below. According to the continuous casting method, when a cast slab having a rectangular cross sectional shape is cast by using a bow-type or a vertical-bending type continuous casting machine, a secondary cooling of the cast slab is carried out immediately after drawing out the cast slab from a mold to cool down the surface temperature of the cast slab one time so that it reaches a temperature lower than the Ar3 transformation temperature. After the secondary cooling has finished, the cast slab is reheated to a temperature exceeding the Ar3 transformation temperature. After that, the cast slab is straightened. In particular, the secondary cooling of the cast slab is carried out satisfying the following Formulae (1) and (2):50≦t(s)≦500   (1)0.13t+493≦Tmin(° C.)≦0.045t+798   (2)
wherein, t(s) indicates a time for holding the surface temperature of the cast slab to the temperature lower than the Ar3 transformation temperature, and Tmin(° C.) indicates the lowest surface temperature which the surface temperature of the cast slab can reach while the cast slab is reheated to a temperature exceeding the Ar3 transformation temperature after it is cooled down one time to a temperature lower than the Ar3 transformation temperature. According to the secondary cooling, a solidification structure from the surface of the cast slab to at least a depth of 2 mm is composed of a mixed structure of ferrite and pearlite of which the grain boundary of the austenite is not clear.
However, according to the above-mentioned methods, the following problems may occur.
According to the method for restraining surface cracks of a cast slab obtained by continuously casting steel described in Japanese Unexamined Patent Application, First Publication No. H09-47854, a cast slab is drawn out from a mold and the cast slab is immediately subjected to a secondary cooling to cool down the surface temperature of the cast slab to the A3 transformation temperature or lower within 1 minute. However, the present inventors have found that, for example, it is impossible to prevent the cracking of the cast slab at the bending point and the straightening point even when the cast slab is cooled down to 725° C. which is the lowest temperature among the temperatures disclosed in the Examples of Japanese Unexamined Patent Application, First Publication No. H09-47854. It is considered that the reason is because it was impossible to refine a structure of the surface portion of the cast slab.
According to the continuous casting method described in Japanese Unexamined Patent Application, First Publication No. 2002-307149, t(s), which indicates a time for holding the surface temperature of the cast slab to the temperature lower than the Ar3 transformation temperature, and Tmin(° C.), which indicates the lowest surface temperature which the surface temperature of the cast slab can reach while the cast slab is reheated to the temperature exceeding the Ar3 transformation temperature after it is cooled down one time to a temperature lower than the Ar3 transformation temperature, are limited to a predetermined range. According to this method, it is possible to prevent surface cracks in the cast slab.
In general, the cooling of a cast slab is classified broadly into cooling by a roll which is in contact with the cast slab and cooling by water or a mixture of water and air discharged from a nozzle disposed between the rolls. However, in a secondary cooling zone right under a mold, the cast slab is not in contact with the rolls and there is a region in the cast slab where the water or the mixture of water and air does not reach, thereby increasing the surface temperature in this region.
Accordingly, even when the cast slab is cooled down one time to a temperature lower than the Ar3 transformation temperature, the cast slab is immediately reheated to a temperature exceeding the Ar3 transformation temperature. Therefore, it is extremely difficult to consistently hold the cast slab to the temperature not greater than the Ar3 transformation temperature for 50 seconds or longer with the general cooling facility. Because of the above-mentioned reason, the continuous casting method described in Japanese Unexamined Patent Application, First Publication No. 2002-307149 is not realistic from the industrial viewpoint.
Therefore, the present invention is to provide a continuous cast slab of Ni-added steel produced by using a vertical-bending type or a bow-type continuous casting machine, in which the appearance of surface cracks is restrained, and to provide a producing method therefor.